Create two formula fields in the main report to be used for passing the params to any sub report. One for StartDate2 and one for EndDate2
@StartDate2 will be
{?StartDate}
@EndDate2 will be
{?EndDate}
Now right click on the sub report and select Change Sub Report Links
Select @StartDate2 and move it into Fields to link on
Select @EndDate2 and move it into Fields to link on
Select OK
Go into the sub report.
You should now see 2 new params
Go into the select expert and use them to limit your data